If i comment the play() line : //play() the error goes away.
If i uncomment, it's back.

Here's some copy/paste from eclipse log windows:

**********************************************************
Problems occurred when invoking code from plug-in: "org.eclipse.core.resources".
**********************************************************
java.lang.ClassCastException: org.eclipse.wst.jsdt.internal.core.CompilationUnit cannot be cast to org.eclipse.wst.jsdt.internal.core.builder.SourceFile
at org.eclipse.wst.jsdt.internal.core.builder.AbstractImageBuil der.acceptResult(AbstractImageBuilder.java:122)
at org.eclipse.wst.jsdt.internal.compiler.Compiler.compile(Comp iler.java:353)
at org.eclipse.wst.jsdt.internal.core.builder.AbstractImageBuil der.compile(AbstractImageBuilder.java:286)
at org.eclipse.wst.jsdt.internal.core.builder.BatchImageBuilder .compile(BatchImageBuilder.java:86)
at org.eclipse.wst.jsdt.internal.core.builder.AbstractImageBuil der.compile(AbstractImageBuilder.java:225)
at org.eclipse.wst.jsdt.internal.core.builder.BatchImageBuilder .build(BatchImageBuilder.java:58)
at org.eclipse.wst.jsdt.internal.core.builder.JavaBuilder.build All(JavaBuilder.java:291)
at org.eclipse.wst.jsdt.internal.core.builder.JavaBuilder.build (JavaBuilder.java:199)
at org.eclipse.core.internal.events.BuildManager$2.run(BuildMan ager.java:629)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.core.internal.events.BuildManager.basicBuild(Bui ldManager.java:172)
at org.eclipse.core.internal.events.BuildManager.basicBuild(Bui ldManager.java:203)
at org.eclipse.core.internal.events.BuildManager$1.run(BuildMan ager.java:255)
at org.eclipse.core.runtime.SafeRunner.run(SafeRunner.java:42)
at org.eclipse.core.internal.events.BuildManager.basicBuild(Bui ldManager.java:258)
at org.eclipse.core.internal.events.BuildManager.basicBuildLoop (BuildManager.java:311)
at org.eclipse.core.internal.events.BuildManager.build(BuildMan ager.java:343)
at org.eclipse.core.internal.events.AutoBuildJob.doBuild(AutoBu ildJob.java:144)
at org.eclipse.core.internal.events.AutoBuildJob.run(AutoBuildJ ob.java:242)
at org.eclipse.core.internal.jobs.Worker.run(Worker.java:54)